2. Principle of three-phase servo transformer: When AC servo motor has no control voltage, there is only pulsating magnetic field generated by excitation winding in the stator, and the rotor is stationary. When there is a control voltage, a rotating magnetic field is generated in the stator and the rotor rotates in the direction of the rotating magnetic field. Under the condition of constant load, the speed of the motor changes with the control voltage. When the phase of the control voltage is opposite, the servo motor will reverse and output a stable point voltage.